The present invention generally relates to semiconductor devices and more particularly to a flip-chip semiconductor device in which a semiconductor chip is mounted on a substrate in a face-down state.
With increasing degree of device miniaturization in the field of semiconductor technology, there has been a need to provide a sufficient number of contact leads when the semiconductor device is assembled in the form of a semiconductor package.
In view of the foregoing needs, there is a BGA package structure in which a semiconductor chip carrying thereon bump electrodes in correspondence to the electrodes of the chip are mounted on a package substrate in a face-down state. The semiconductor chip thus flip-chip mounted on the package substrate is covered by a cap member, which functions also as a heat sink. The package substrate, on the other hand, carries further bump electrodes on a side thereof opposite to the side on which the semiconductor chip is flip-chip mounted, and the semiconductor device thus formed of the package substrate is mounted on a main substrate, which may be a printed circuit board of an electronic apparatus.
FIG. 1 shows the construction of a BGA semiconductor device 10 according to a related art.
Referring to FIG. 1, the semiconductor device 10 includes a package substrate 11 on which a semiconductor chip 12 is flip-chip mounted, such that bump electrodes 12A typically of a Snxe2x80x94Ag alloy and formed on the bottom surface of the semiconductor chip 12 make a contact engagement with a corresponding wiring pattern (not shown) provided on a top surface of the package substrate 11. Further, bump electrodes 11A are provided on a bottom surface of the package substrate 11 in electrical connection with the wiring pattern provided on the top surface, wherein the bump electrodes 11A may be formed of a solder bump. The bump electrodes 11A form a ball grid array on the bottom surface of the package substrate 11.
The semiconductor chip 12 thus flip-chip mounted on the package substrate 11 is then covered by a cap member 13, wherein the cap member 13 is thermally connected to the semiconductor chip 12 via a thermally conductive adhesive layer 13A such as a silver paste. The cap member 13 thereby functions as a heat sink. The cap member 13 is also fixed to the top surface of the package substrate 11 mechanically by an adhesive layer 13B such as an epoxy resin. Further, a resin layer 12B, typically of an epoxy resin, fills the space formed between the bottom surface of the semiconductor chip 12 and the top surface of the package substrate 11, in which space the bump electrodes 12A make a contact engagement with the wiring patterns on the package substrate 11. By filling the space with the resin layer 12B, the reliability of electrical contact of the bump electrodes 12A is improved.
It should be noted that the BGA semiconductor device 10 is then mounted on a main substrate 14, which may be a printed circuit board of an electronic apparatus. Typically, the package substrate 11 is formed of a multilayer ceramic substrate or a multilayer resin substrate. On the other hand, the cap member 13 acting as the heat sink is formed of a thermal conducting material such as Cu, Al, Alxe2x80x94SiC or AlN.
In the device 10 of FIG. 1, a typical cap member 13 is formed of an Alxe2x80x94SiC composite having a Young modulus of 110 GPa. In this case, the thermal expansion coefficient of the cap member 13 has a value of 1.2xc3x9710xe2x88x925/xc2x0 C. In conformity with the thermal expansion coefficient of the cap member 13, the package substrate 11 may be formed of a glass ceramic having a thermal expansion coefficient of 1.2xc3x9710xe2x88x925/xc2x0 C. The glass ceramic substrate 11 typically has a Young modulus of 70-75 GPa. On the other hand, the main substrate 14 is typically formed of a glass-epoxy resin and has a thermal expansion coefficient of 1.6-1.7xc3x9710xe2x88x925/xc2x0 C.
In the actual use of the BGA semiconductor device 10 in an electronic apparatus, the semiconductor device 10 is subjected to a thermal cycle process associated with turning-on and turning-off of the electronic apparatus. Thereby, such a temperature cycle induces a thermal stress in the semiconductor device 10 particularly in correspondence to the part where the bump electrodes 11A make an electric contact with corresponding wiring patterns on the main substrate 14, and there is a risk that the contact fails as a result of fatigue.
This problem of fatigue appears conspicuously when the difference in the thermal expansion coefficient between the BGA semiconductor device 10 and the main substrate 14 is large. Further, the problem of foregoing thermal fatigue appears conspicuously when the rigidity is increased for the semiconductor device 10. Further, the problem of foregoing thermal fatigue appears conspicuously when the lateral size of the semiconductor device 10, and hence the distance across the outermost bump electrodes on the package substrate 11 is increased.
In view of the fact that the number of input/output terminals is increasing in the advanced high-performance semiconductor devices of these days, the number of the bump electrodes 11A on the package substrate 11 is now reaching the order of several hundreds to several thousands. Associated with this, the distance across the outermost bump electrodes is also increasing and the reliability of the electrical contact has become a serious problem in these advanced, high-performance semiconductor devices.
FIG. 2 shows another BGA semiconductor device 10A according to a related art, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted.
Referring to FIG. 2, the BGA semiconductor device 10A has a similar construction as the BGA semiconductor device 10 of FIG. 1, except that the cap member 13 is replaced with a cap member 23 having a reduced thickness. Thereby, the rigidity of the BGA semiconductor device 10A is reduced as compared with the BGA semiconductor device 10 and the problem of the fatigue of the bump electrodes 11A is reduced.
On the other hand, in view of the fact that the thickness of the cap member 23 is reduced, the cap member 23 no longer functions as an effective heat sink and the semiconductor device 10A of FIG. 2 suffers from the problem of abnormal operation and abnormal temperature rise associated with poor cooling. In the case of the device of FIG. 1, the cap member 13 has a thickness of about 2 mm at the top part contacting the semiconductor chip 12, while the cap member 23 in the semiconductor device 10A of FIG. 2 has a thickness of only 0.3 mm.
Accordingly, it is a general object of the present invention to provide a novel and useful semiconductor device wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a BGA semiconductor device having a package substrate on which a semiconductor chip is flip-chip mounted, wherein the reliability of contact with an external substrate is improved for the electrode bumps that are provided on the package substrate, while maintaining an excellent heat dissipation performance.
Another object of the present invention is to provide a semiconductor device, comprising:
a package substrate;
a semiconductor chip mounted on a top surface of said package substrate in a face-down state;
a cap member provided on said top surface of said package substrate so as to cover said semiconductor chip, said cap member making a contact with said semiconductor chip and said top surface of said package substrate; and
electrodes provided on a bottom surface of said package substrate,
said cap member having a thermal conductivity not smaller than about 100 W/(mxc2x7K) and a Young modulus not exceeding about 20 GPa.
According to the present invention, the rigidity of the semiconductor device as a whole is reduced as a result of the use of the cap member having a Young modulus not exceeding about 20 GPa, and the problem of mechanical fatigue caused in the electrodes on the bottom surface of the package substrate due to the difference of thermal expansion coefficient between the semiconductor device and a substrate on which the semiconductor device is mounted is reduced. As the cap member maintains a high thermal conductivity, the decrease of the rigidity of the semiconductor device does not causes the problem of poor thermal dissipation.
Preferably, the cap member has a thermal expansion coefficient equal to or smaller than 3xc3x971031 5/xc2x0 C. The cap member may be formed of a metal-infiltrated carbon composite material. Further, the cap member may be formed of a directional carbon/metal composite material having a reduced modulus of elasticity in a lateral direction thereof as compared with a thickness direction thereof.